Expression of three members of the ErbB-family of receptor tyrosine kinases, namely EGFR, ErbB2, and ErbB3 are associated with aggressive metastatic breast cancer. In contrast, to the mitogenic activities associated with the other three ErbB-family members, ErbB4 mediates breast epithelial differentiation and ErbB4 expression is selectively extinguished in advanced breast cancer. Furthermore, ErbB4-signaling has antiproliferative activity in breast cancer and actively curtails mitogenic signaling by the protooncogene and therapeutic target, ErbB2. Collectively, these observations support the hypothesis that ErbB4-induced differentiation pathways in breast cancer antagonize the activities of important breast oncogenes, including ErbB2. The following specific aims were designed to test this hypothesis. Aim 1) investigate the influence of constitutive active ErbB4 (ErbB4-CA) on differentiation and ErbB2-induced cellular proliferation in human breast cancer cell lines, Aim 2) develop a preclinical transgenic mouse model of ErbB4-CA overexpression and determine the impact of ErbB4-CA on normal breast development and ErbB2-induced metastatic breast cancer, Aim 3) develop an independent preclinical mouse model to investigate the influence of extinguished ErbB4-signaling on spontaneous and ErbB2-induced breast carcinogenesis and metastasis, Aim 4) support preclinical models by determining the association of ErbB4 expression with clinical outcome and oncogenic phospho-ErbB2 expression in human primary breast tumors. The experiments described in this proposal are designed to provide a thorough analysis of ErbB4 function in the developing breast and determine the influence of ErbB4 signaling on ErbB2-induced metastatic breast cancer. If evidence suggests that ErbB4 activity antagonizes the development and progression of metastatic breast cancer in these preclinical and clinical models, future experiments will be designed to investigate the role of ErbB4 as a putative tumor suppressor in breast cancer and by extension explore the efficacy of ErbB4 signaling pathways as therapeutic agents in breast cancer.